Friction materials, for friction in liquid medium, have been developed particularly in order to respond to requirements of stability, heat resistance and wear.
The patent document EP O 702 761 divulges a friction material in liquid medium including a mat of fibers impregnated with a heat-hardening type of resin, where the mat of fibers consists of fibers at least 12 mm long, and which are carded, lapped and needled (or needle-punched, or felted, or needle-felted).
The porosity of such a friction material may be between 30% and 60% in volume. This gives a material suited to applications in transmission coupling parts of a motor vehicle for classical conditions of use.
Generally speaking, in the present description overall, when one refers to a range as “included between A and B”, the terminals “A” and “B” are generally included in the range.
In a motor vehicle transmission coupling part of classical conditions, the pressures applied on this type of material generally vary between 0.2 and 2 MPa.
The developments of the motor vehicle market are particularly leading designers to improve the efficiency of the drivetrain, for example in order to reduce consumption and/or enhance the comfort of a vehicle.
One of the chosen ways to improve these characteristics consists of automating the transmissions, for example by using automatic transmissions or robotised gearboxes.
Among the numerous criteria for optimising a transmission, one may quote increases of performance, the size of the transmission part, and its cost of manufacture.
Also, a current development trend concerns the possibility of obtaining automated gear changes without pausing.
It has been possible to establish that known friction materials, for friction in liquid medium, for example of the type of the one divulged in the document EP 0 702 761, present properties that make it difficult to jointly optimise the criteria set out above.
It has also been possible to determine that a resistance of the friction material at specific pressures of between at least 0.5 and 10 MPa would be of a nature such as to offer new and promising means of designing transmission parts.